"Gravity Tractor" Could Deflect Earth-Bound Asteroids

A spacecraft could use a gravity "towline" to alter the course of an Earth- bound asteroid, a new study by two NASA astronauts suggests.

Previous schemes to deflect an incoming space rock range from landing a spacecraft on the asteroid and pushing it off course to blowing it up with nuclear weapons.

The new plan takes a gentler approach. A spacecraft would hover above the asteroid and gradually pull it off course using nothing more than the gravitational attraction between the two bodies.

"If an asteroid is found to be at an impact trajectory with Earth you will have many decades of notice. And it turns out that you only need to change its velocity by a very small amount in order to prevent a collision," said Edward Lu, a NASA astronaut at the Johnson Space Center in Houston, Texas.

Lu devised the plan with fellow NASA astronaut Stanley Love. The pair report their findings in tomorrow's issue of the science journal Nature.

Nuclear Option

Formed during the creation of the solar system, most asteroids are made of rock, while about 3 percent are made of metals like iron. Most orbit the vast region of space between Mars and Jupiter.

The probability of a large asteroid hitting Earth is extremely slim. But if one did strike, it could cause widespread damage. Because of this scientists have been looking for ways to deflect asteroids from potential collision courses with Earth.

One scheme proposes attaching a spacecraft to an asteroid and firing the craft's engines to push the space rock away.

The plan presents some practical difficulties, partly because asteroids spin as they travel through space. The task is also made harder because scientists are unsure what the surface of an individual asteroid looks like.

"Landing means dealing with a rough surface with poorly known physical properties and somehow compensating for the asteroid's rotation, which wants to whirl your thrust[ers] around like a lawn sprinkler," Love said. "Using gravity as a towline frees you from those messy details."

Lu adds that blowing up an asteroid with nuclear weapons or slamming a large spacecraft into it to break it up are "bad ideas."

"If you do that, you better have a darn good idea where all those pieces are going," Lu noted.

Gravity Tractor

With the gravity plan, a spacecraft would not have to dock on the asteroid, but instead hover above its surface. The craft's thruster jets would angle outward to avoid blasting the asteroid's surface and pushing it away.

"You would use this small gravity force between the [spacecraft and the asteroid] as your towline to basically pull this thing," Lu explained.

"We had looked at landing on the surface [of an asteroid] and all the problems associated with that, when we found that the gravitational force is about equal to the force that you plan to be pushing with anyhow," he said. "We realized, Wow, rather than pushing, why don't we just pull?"

The astronauts calculate that, given a lead time of about 20 years, NASA could launch a spacecraft that could safely deflect an asteroid some 650 feet (200 meters) across in about a year of "towing."

Lu notes that early intervention makes the job easier: When an asteroid has a close Earth approach, followed by a later return and possible impact, the gravitational tug needed to prevent the impact would be much smaller if applied before the close approach.

This scenario applies to 99942 Apophis, a 1,000-foot (320-meter) asteroid that will swing by Earth at a distance of about 19,000 miles (30,000 kilometers) in 2029. The flyby will change the asteroid's orbit, and there is a small chance (about 1 in 5,000) that it will hit Earth when it returns in 2036.

"If around 2013 we find out that it's going to hit, we could [initiate] a very tiny change in [the asteroid's] orbit before 2029," Lu said.

Love says he personally doesn't lose much sleep over the possibility of a major asteroid impact, but that it would be foolish to put off making plans for such an event until the last minute.

"In my office, we do not wait until the building is on fire to conduct fire drills," the astronaut said. "Recent large natural disasters, such as the tsunami in the Indian Ocean and the hurricanes in our own country, underscore the value of being prepared for an emergency."